This invention relates to an optical power control device for a semiconductor light emitting element.
One example of a conventional optical power control device is shown in FIG. 1. A laser diode 1, which is a semiconductor light emitting element, is employed as a light source for emitting a light beam for writing data on or reading data from a recording medium. A monitor diode 2 is built into the laser diode 1. The monitor diode 2, which acts as a photodetector, receives part of the output laser beam of the laser diode.
The output of the monitor diode 2 is applied through a monitor amplifier 3 to a sample and hold circuit 4. The sample and hold circuit 4 includes a switch SW.sub.1 connected to the output terminal of the monitor amplifier 3 and a capacitor C1 connected between the output terminal of the switch SW.sub.1 and ground. In response to a write gate signal which is outputted by a controller (not shown) during a data writing interval, the switch SW.sub.1 is turned off (or opened) in a write mode, and is turned on (or closed) in a read mode.
The output of the sample and hold circuit 4 is applied to the inverting input of an operational amplifier OP which forms an integrator 5 together with a capacitor C.sub.2. A voltage corresponding to a read power set value provided by a read power setting circuit 6 is applied to the non-inverting input terminal of the operational amplifier OP. so that the integrator 5 outputs a voltage corresponding to the difference between the output voltage of the sample and hold circuit 4 and the voltage corresponding to the read power set value. The output voltage of the integrator is applied to a V-I (voltage to current) converter 7, where it is converted into a current which is applied through an adder 8, as a write current, to the laser diode 1.
The laser power must be greater in the data writing mode than in the data reading mode. The laser power for writing is set by a write power setting circuit 9, which outputs a voltage corresponding to the write power set value. The output voltage of the write power setting circuit 9 is applied to a V-I (voltage to current) converter 10, where it is converted into a current which is supplied to a switch SW.sub.2.
In the data reading mode, the armature of the switch SW.sub.2 is connected to one contact which is grounded, and in the data writing mode the switch is turned on and off in accordance with the data which is to be written. The output current of the switch SW.sub.2, being superposed, as a write current, on the read current at the adder 8, is supplied to the laser diode 1.
The above-described sample and hold type optical power control device is designed so that, in the data writing mode, .the optical power detection voltage of the monitor diode in the data reading mode occurring immediately before the data writing mode is stored in the capacitor C.sub.1, and the voltage corresponding to the write power set value is added to the stored voltage. Therefore, no trouble results in the case of a recording format in which, as in a data recording operation, there is a switchover from writing power to reading power.
However, the above-described device is disadvantageous in that, for instance in recording data over a long interval, it is impossible to hold the optical power detection voltage in the data reading mode because of the discharge of the capacitor C.sub.1. and therefore it is impossible to instantaneously switch the data writing mode over to the data reading mode.
Instead of the sample and hold circuit 4, analog-to-digital (A/D) and digital-to-analog (D/A) converters may be employed. However, even with such structure, if there is a long data writing time, then because of heat generated by the laser diode 1, the current vs. optical power characteristic curve changes. As a result, it is impossible to hold the optical power detection voltage obtained in the data reading mode. Thus, similarly as in the above-described case, it is impossible to instantaneously switch the data write mode over to the data read mode.
Another example of a conventional optical power control device is disclosed in Japanese Unexamined Published Patent Application (OPI) No. 146457/1984. In the conventional device, the average value of write data is obtained, and the product of the average value and the write power set value is subtracted from the optical power detection voltage of the photodetector in order to stabilize the system. However, such a device is deficient in that, since it is designed to use a multiplier for obtaining the product of the write data average value and the write set value, drift may occur, and offset adjustment may be required.